COMPOSITES

Sticky rice–nanolime as a consolidation treatment

for lime mortars

J. Otero1,2,* , A. E. Charola3, and V. Starinieri1

1Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UK2Getty Conservation Institute, The J. Paul Getty Trust, Los Angeles, CA 90049, USA3Museum Conservation Institute, Smithsonian Institution, Washington, DC, USA

Received: 14 February 2019

Accepted: 9 April 2019

Published online:

17 April 2019

� The Author(s) 2019

ABSTRACT

For almost 1500 years, many ancient Chinese mortars have remained unaltered

despite exposure to atmospheric agents. The main reason for this long-term

durability is the addition of sticky rice water to the standard mortar ingredients

(lime and sand) following traditional recipes. In recent years, these mortars have

been methodically studied leading to the conclusion that amylopectin, a

polysaccharide in the sticky rice, plays a crucial role in regulating calcite crystals

growth, creating a denser microstructure and providing the mortar with

hydrophobic properties which contributed to their survival. In recent decades,

nanolime products based on Ca(OH)2 nanoparticles suspended in alcohol or

hydro-alcoholic medium have been extensively used for the consolidation of

calcareous substrates mainly due to their chemical affinity and absence of side

effects. Nanolime products have resulted in successful superficial consolida-

tions. However, in-depth consolidation still needs to be achieved, and research

needs to focus on ways to attain this objective. This study aimed to test a novel

approach consisting of applying a pre-treatment of sticky rice and subsequently

the nanolime. The resulting consolidation was evaluated by measuring changes

of superficial cohesion, porosity, contact angle, drilling resistance, water

absorption by capillarity, drying rate and aesthetic properties. The durability of

the treatments was investigated by exposing samples to accelerated weathering.

Results showed that the use of sticky rice in combination with nanolime yields a

higher degree of consolidation increasing drilling resistance and delivering

hydrophobic properties although prolonged exposure to high temperature and

moisture can compromise treatment durability.

Address correspondence to E-mail: Jotero@getty.edu; jorge.otero.h@gmail.com

https://doi.org/10.1007/s10853-019-03618-1

J Mater Sci (2019) 54:10217–10234

Composites

Introduction

Lime mortars have been used since historic times and

to improve their texture and durability organic

additives were included [1]. For example, oil from

various sources, i.e., olive, linseed, or tallow, to make

them water-repellent especially for limewashes;

casein or blood as plasticizers; sugar, as setting

retardants; egg white [2] to improve workability, and

hair to increase their mechanical resistance. The

selection of additives depended on the availability of

the product, so olive oil was used in Mediterranean

countries, such as Portugal and Italy, sugar in Brazil,

and tallow in countries where sheep or cattle were

abundant. Thus, each region developed its own

specific tradition; in China, sticky rice was the pre-

ferred additive, this being the water in which sticky

rice was cooked [3]. Rice contains two types of starch,

amylopectin that makes rice sticky, and amylose that

does not gelatinize; approximately 70% w/w of the

starch is amylopectin, while amylose is only 5%

w/w, and protein is also found, between 6.8 and 9.6%

w/w, that could apparently influence the carbonation

of the lime [4]. Amylopectin is a branched-chain

polysaccharide composed of glucose units linked

primarily by a - 1,4-glycosidic bonds but with

occasional a - 1,6-glycosidic bonds, while amylose is

a linear polysaccharide composed entirely of D-glu-

cose units joined by the a - 1,4-glycosidic linkages

[5, 6].

The sticky rice mortars were used in many histor-

ical buildings completed during the Ming

(1368–1644) and the Qing (1644–1911) dynasties, such

as the dyke at the Hangzhou Bay where the Quiantan

river drains or the Jingzhou Historic Town Wall, one

of the best preserved and largest historic town walls

in Southern China that survived to date, [3, 7–11]. The

earliest record of sticky rice–lime mortar was found

in the ancient building book Tian Gong Kai Wu which

was written during the Ming Dynasty (1368–1644

AD) [12]. However, there are archaeological evidence

that show that they were also used during the South-

North Dynasty (386–589 AD) [13]. According to

ancient Chinese manuscripts, lime mortars used in

those structures also contained sticky rice, although

the formula for preparing the latter was not clear [14].

Although the sticky rice/lime mortar technology was

well-known, these mortars had never been scientifi-

cally analyzed to understand the reasons for their

good performance until the last decade. In 2008,

several samples were prepared according to historic

records of Chinese ancient books to determine the

sticky rice/lime proportions and compared to sam-

ples taken from historic structures [3]. In this study, it

was concluded that the microstructure of the pre-

pared samples was very similar to that of ancient

mortar samples, especially those containing a 3%

volume ratio of sticky rice solution. This preliminary

study also suggested that that sticky rice could play a

crucial role in the microstructure and resistance of

lime mortar as it functions as a template controlling

the growth of calcium carbonate crystals. In subse-

quent years, many studies followed addressing the

sticky rice/lime proportions [4, 8, 15, 16] and focus-

ing on both the setting mechanism and the effect it

had on improving the mechanical resistance

[4, 7, 8, 15–18]. A systematic study of sticky rice–lime

mortar technology was carried out varying sticky rice

and lime content [15]. The physical properties, such

as mechanical strength, were found to be significantly

improved, especially those with 3% sticky rice solu-

tion. The authors also confirm that amylopectin, the

main component of the sticky rice, acted as an inhi-

bitor controlling the crystal growth of calcium car-

bonate creating a denser microstructure with smaller

calcium carbonate crystals, explaining the good per-

formance of these mortars. Other researchers found

that the optimal sticky rice proportion when added to

the lime mortar could vary from 1 to 3% [4] to 5% of

sticky rice [7]. Further studies also showed that the

formation of a more compact microstructure of

smaller sized calcium carbonate crystals increases the

mechanical properties, as calcium carbonate crystals

are covered with a layer of sticky rice thus improving

their resistance to weathering processes

[4, 9, 10, 17–19]. The covering of the calcite crystals by

the sticky rice in the mortar was also found to reduce

the penetration of water in the structure and increase

its resistance to wet–dry cycles [8, 18]. Additionally,

this feature contributed to the higher seismic resis-

tance of the mortars, as illustrated by the many

ancient stupas, temples and bridges in Quan County

built with sticky rice–lime mortar which survived the

7.5 earthquake of 1604 A.D [20].

Most of these studies differ in how the sticky lime

mortars were reproduced, as they varied significantly

from the traditional approach where sticky rice

solution was added to the quicklime during

quenching: ‘‘Usually, lime reacts with water to

10218 J Mater Sci (2019) 54:10217–10234

generate hydrated lime with emission of a lot of heat

and simultaneously create alkaline environment that

can effectively suppress and kill bacteria’’ [7, 10] thus

explaining why sticky rice did not undergo biodete-

rioration over time. Most studies use commercial

hydrated lime [4, 8, 16], or slaked lime [15], while

others prepared the ‘‘lime’’ solution by reacting a

calcium chloride (CaCl2) solution with Na(OH) [19]

and in some cases using a supersaturated solution of

this mixture and artificially carbonating it [3].

Nonetheless, most of the papers agree that the addi-

tion of sticky rice solution contributes significantly to

the increase of the mortar’s setting, mechanical

strength, bonding capability, and reduced water

absorption, though the study by Zhao et al. [16],

which also addressed the addition of tung oil and pig

blood, found that 5% sticky rice slurry ‘‘accelerates

setting and hardening and increases compressive

strength and bonding capability, although unit

weight and water resistance are slightly reduced.’’

The fairly recent development of nanotechnology

has brought new nanomaterials with potential supe-

rior properties. Nanolimes (dispersions of Ca(OH)2nanoparticles in alcohol or hydro-alcohol medium),

were developed in the last decades as an effective

and compatible conservation material for historic

calcareous materials. Due to its high reactivity and

compatibility, nanolimes resulted in successful

superficial consolidation of historic substrates such as

wall-paintings [21–23]. However, when an in-depth

consolidation is required, their effectiveness is con-

troversial since rarely do nanolimes attain penetra-

tions above 1 cm [24]. Some studies focused on

different ways to improve the nanolime in-depth

consolidation effectiveness, for example, Daniele

et al. [25] achieved faster and deeper consolidation by

adding sodium bicarbonate (NaHCO3) to the nano-

lime alcohol solution. However, the addition of

NaHCO3 may induce the formation of salt efflores-

cence. Furthermore, other studies focused on creating

a CO2-rich atmosphere in a yeast–sugar environment

to accelerate the carbonation process and to increase

the in-depth carbonation [26]. Recently, other studies

suggested that the type of solvent could be critical for

the nanolime effectiveness in highly porous sub-

strates [27–29]. These studies suggest that solvents

with low evaporation rate could be more suitable for

coarse porous substrates as they would reduce the

back-migration of the nanoparticles thus contributing

to their in-depth deposition. However, more research

needs to be carried out to elucidate the correlation

between solvent, pore size distribution and penetra-

tion depth. From the current literature, it is clear that

the consolidation effect of nanolimes still requires

further study and more research needs to be carried

out to study new ways to increase the nanolime in-

depth consolidation effectiveness.

The present study focused on the effect sticky rice

had when used as a pre-treatment before applying

nanolime for consolidation of lime mortars. Physical

properties, mechanical strength, penetration depth,

water absorption, hydrophobicity and durability of

samples treated by the combined treatment is com-

pared with that of samples exposed to single treat-

ments of either nanolime or sticky rice. This is an

innovative approach and the results obtained are

promising but several issues have as yet to be

elucidated.

Materials and methods

Lime mortar samples

Lime mortars were prepared using a dry singleton

Birch Ultralime CL90 and fine crushed silica sand

(0.07\Ø\ 1.5 mm, Pentney, UK) in a binder/ag-

gregate ratio of 1:2.5. Mortars were batched by weight

after measuring the component densities in accor-

dance with EN 1015-2:1998 [30]. The water/binder

ratio was 1.56 to obtain a constant flow of 16 cm

measured according to BS EN 1015-3:1999 [31]. The

mixing and casting procedure of the mortar can be

found elsewhere [24]. Mortar beams

(40 9 40 9 160 mm) were de-molded after 5 days

and then stored in an outdoor sheltered area

(T & 5–15 �C, R.H & 60–80%) for 28 curing days.

Upon completion of the curing time, each prism was

cut into approximately 4 cubes of 40 9 40 9 40 mm

prior to curing for another 28 days in the same

conditions.

The mineralogical composition of the mortar was

obtained by X-ray diffraction (PANalytical XPert

PRO) recorded with a 0.0262h step size in the angular

range 20–70�2h. X-ray data were refined by means of

Rietveld refinement [32, 33]. The experimental

diffraction pattern was elaborated by means of a

Profile Fit Software (HighScorePlus, PANalytical),

and each crystalline phase was identified using the

ICSD and ICDD reference databases. The XRD

J Mater Sci (2019) 54:10217–10234 10219

samples were ground and sieved through an 80 lmsieve mesh and placed over an XRD zero-background

sample holder. The XRD-Rietveld refinements show

the mortar to be composed of 79.9% Quartz (SiO2,

ICSD #00-046-1045) and 20.1% calcite (CaCO3, ICSD

#00-005-0586).

The pore structure of the mortar was determined

by Mercury Intrusion Porosimetry (MIP) using a

PASCAL 140/240 instrument. The samples for MIP

were mortar fragments measuring approximately

8 9 15 mm taken from the surface (up to a depth of

50 mm) of the sample, which were dried in an oven

at 60 �C to constant weight. The average porosity of

the mortar measured by MIP was 23 ± 1.6 vol% with

a bulk density of 1.698 ± 0.1 g/cm3.

Nanolime

Nanolime was synthesized through an anionic

exchange process carried out at room temperature

and ambient pressure by mixing under moderate

stirring an anion exchange resin (Dowex Monosphere

550A OH by Dow Chemical) with an aqueous cal-

cium chloride solution (CaCl2 by Sigma-Aldrich), as

described in the literature [34, 35]. After the synthe-

sis, the newly synthetized nanoparticles of Ca(OH)2were dispersed in a 50–50% isopropanol/water sol-

vent maintaining the concentration at 5 g/L, as this

was found to be the optimal alcohol/water ratio

[24, 35]. A description of morphology, reactivity and

colloidal stability of this nanolime can be found in the

latter reference.

Sticky rice

Commercial pulverized sticky rice was used in this

study (Indica type, Enuo 6 variety) and was pur-

chased from Erawan Marketing Co. TLD as ‘‘Farine

de Riz Gluant’’ (product of Thailand). A well-known

approach was followed to prepare the sticky rice

starch suspension [4, 15, 36]. First, 2 g of pulverized

sticky rice are added to 400 mL of deionized water

(5 wt%) and left to soak for an hour. The suspension

was then gradually heated (up to 300 �C) and stirred

for 4 h. Afterward, the resulting paste was left to cool

to T * 30 �C under vigorous stirring for approxi-

mately 1 h. The suspension was applied immediately

after preparation.

Characterization of sticky rice

The compositions of the sticky rice powder and the

prepared sticky rice paste suspension were studied

by ATR-FTIR (Thermo Nicolet Nexus instrument).

The dry sticky rice powder was ground

(Ø\ 150 lm), dried to constant mass at 60 �C and

placed directly on the diamond crystal of the ATR

accessory. For the sticky rice paste, a drop of the

suspension was placed directly on the diamond

crystal of the ATR accessory. FTIR spectra were col-

lected by 64 scans in the range 400–4000 cm-1 at a

spectral resolution of 4 cm-1. A piece of aluminum

foil was used to back the sapphire anvil to eliminate

any sapphire absorption in the IR spectrum. The

obtained FTIR spectrum was identified using the

IRUG (Infrared and Raman Users Group) libraries

[37], the HR Hummel Polymer and Additives library

as well as the ASTER mineral library.

To further study the composition of the sticky rice,

a starch–iodine test was conducted. The iodine

reagent (KI) was prepared by dissolving 0.2 g of

iodine in 100 mL of deionized water in the presence

of 2.0 g of KI. Drops of the iodine-KI reagent were

introduced into the sticky rice suspension. Sticky rice

starch is a mixture of two types of polysaccharides:

amylose and amylopectin [5]. The suspension turns

blue in the presence of amylose or red in the presence

of amylopectin [38].

The morphology of the sticky rice paste was

observed by SEM (NOVA NANOSEM 200 instru-

ment). SEM samples were prepared by placing a few

drops (± 0.10 mL) of sticky rice suspension on a

carbon film on a copper SEM sample holder. SEM

micrographs were taken with an ETD detector, a

working distance of & 3 mm, an accelerating voltage

of & 15 kV and a spot size of & 30 nm in high vac-

uum (25 MPa). Specimens were coated with a 20 nm

thick layer of gold using a Quorum Q150T coater unit

at 10 MA sputter current and 420 s sputter time.

The rheological behavior of the sticky rice sus-

pension was measured using a Rheometer (Anton-

Paar Physica MCR 301), since viscosity is an impor-

tant factor for the penetration of a liquid in a porous

material [39]. The measurements were made at 30 �C.The obtained flow curves plot the shear stress (mPa)

versus the velocity gradient (s-1) and were compared

to that of water.

The surface tension of the sticky rice suspension

was determined by the pendant drop method using

10220 J Mater Sci (2019) 54:10217–10234

an OCA 15 Plus instrument (Dataphysics). For this

test, a sticky rice suspension drop (5 lL) was pendant

through a Hamilton 50 lL DS 500/GT syringe. The

surface tension of the sticky rice was calculated by

measuring the shape of the pendant drop of the

dosing needle, defined with the Young–Laplace

equation:

DP ¼ c1

R1þ 1

R2

� �

where ‘‘DP’’ is the difference in pressure across the

interface, ‘‘c’’ the surface tension, and ‘‘R1’’ and ‘‘R2’’

are the curvature radius.

Sticky rice and nanolime application

The sticky rice and nanolime were applied on three

40 9 40 9 40 mm lime–mortar cubes following the

application method described below. Three cubic

mortars were treated with nanolime (LAQ), three

with sticky rice (SR) and three with a combination of

sticky rice and nanolime (SR-LAQ).

The sticky rice suspension was used both as a

consolidant and a pre-treatment prior to the nano-

lime application. As a consolidation treatment, the

sticky rice suspension was applied by brush in lab-

oratory conditions (* 50%RH) on only one face of

the specimen until no further absorption was

observed for a period of at least 5 min between each

brushstroke. The sticky rice was absorbed throughout

the cubic samples reaching the opposite face. Upon

completion the treatment, the samples were dried in

an oven to accelerate water evaporation at 30–40 �Cto constant mass. Samples were weighed before and

after application, once constant weight was reached,

to determine the total amount of the absorbed dry

sticky rice product by each substrate.

As a pre-treatment, the sticky rice solution was

applied following the above description and once the

samples reached constant mass, the nanolime was

immediately brushed on the same face of the speci-

men. The nanolime dispersion was applied 2 days

after its synthesis to increase its effectiveness [40].

The nanolime dispersion was agitated before appli-

cation to increase the colloidal stability of the parti-

cles [24]. The application was ended when no further

absorption was observed (surface remains com-

pletely wet for a period of at least 1 min). The

nanolime was also absorbed throughout the cubic

samples reaching the opposite face. Samples were

also weighed after application to determine the total

amount of nanolime absorbed by each substrate.

A complete description of the application method

of nanolime can be found elsewhere [24]. Upon

treatment completion, the treated samples were

stored for 28 days at outdoor environment conditions

(70 ± 5% RH, T = 10 ± 5 �C). A set of untreated

control samples was stored under the same outdoor

conditions, and are referred to as CO.

Consolidation effectiveness

Following the 28-day outdoor exposure, the treated

and untreated lime mortar cubes were dried to con-

stant mass at 60 �C in a fan-assisted oven and sub-

sequently stored in a desiccator until testing.

Pore size distribution and porosity were measured

by MIP (Pascal Inst.140/240). Tests were carried out

on two samples taken from the surface (up to a depth

of 50 mm) of treated and control mortars. Contact

angle of the mercury and treated samples was taken

to be 140. Samples were dried to constant mass at

60 �C for 24 h prior to the analysis.

Water absorption coefficient (WAC) and capillary

absorption curves were obtained [41]. Upon com-

pletion of this test, the samples were immersed in

water for 24 h and then weighed at room conditions

to calculate the apparent and open porosity [42].

Finally, the drying behavior was also determined

[43]. This test sequence was carried out on three

control specimens and three treated specimens per

each treatment.

Surface cohesion of the specimens after treatment

was evaluated by the ‘‘Scotch Tape Test’’ (STT)

according to ASTM D3359 [44]. The test was per-

formed on both treated and control samples, and

results were taken as the average of 9 measurements

per sample.

The penetration and consolidation effectiveness of

the treatments was also assessed by means of a

Drilling Resistance Measurement System (DRMS)

from SINT-Technology [45, 46]. Tests were per-

formed on both control and treated samples using

drill bits of 5 mm diameter, a rotation speed of

600 rpm, a penetration rate of 15 mm/min and a

penetration depth of 20 mm. Drilling resistance val-

ues were calculated as the mean of 6 tests carried out

on 2 specimens per each treatment.

The hydrophobic behavior of the treatments was

determined by contact angle measurements (OCA 15

J Mater Sci (2019) 54:10217–10234 10221

Plus, Dataphysics Instrument), to determine the

wettability of the substrates after the applied treat-

ments. For this test, a water drop (5 lL, deionisedwater) was placed over the substrate surface using a

Hamilton 50 lL DS 500/GT syringe. Pictures of the

water drop on specimens� surfaces were taken 1 s

after applying the water drop and the contact angle

was calculated by the software. The static contact

angle (h) reported was the average of 10

measurements.

Changes in color were determined by means of a

spectrophotometer (Minolta CM508D Colorimeter)

with the CIE-Lab system [47], using 30 readings taken

in different areas of the surface of each of the treated

and control samples. Total color variation (DE) was

calculated by the formula:

DE ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDL�2 þ Da�2 þ Db�2

pwhere DL*, Da* and Db* are the change in luminosity

for white–black, red–green and blue–yellow param-

eters, respectively.

Durability of the consolidation treatments

The durability of the sticky rice and sticky rice–

nanolime treatments was assessed by exposing three

mortar samples treated with SR and three treated

with SR-LAQ to Accelerated Weathering (AWT,

QUV/SE Accelerated Weathering Tester from Q-Lab

Europe Ltd) under two different exposure conditions:

• AWT 1: Step 1: 8 h UV at 1.55 W/m2 irradiance

and 60 �C; Step 2: 4 h moisture condensation at

50 �C with no UV irradiation (ASTM G154 CYCLE

6). Duration 340 h.

• AWT 2: Step 1: 8 h UV at 1.55 W/m2 irradiance

and 40 �C; Step 2: 4 h moisture condensation at

30 �C with no UV irradiation. Duration 340 h.

To further assess the durability of the treatments,

the resistance to wet–dry cycles was evaluated fol-

lowing a method described in the literature [8]. Three

cubic specimens for both SR and SR-LAQ were used

for this test. During the experiment, all the samples

were immersed in deionized water for 8 h, with the

water level 20 mm over the samples’ upper surface.

After thoroughly soaked, the samples were put into

the fan-assisted oven (40 �C) for 8 h, and then natu-

rally cooled to the normal atmospheric temperature.

This cycle was repeated 20 times.

After both the weathering in the QUV and the wet–

dry test, the durability of the treated samples was

assessed by measuring changes in the mortar’s

superficial cohesion (by Scotch Tape Test), drilling

resistance (DRMS) and hydrophobic properties

(contact angle). A small amount of water used from

the wetting–drying cycles were also analyzed by

FTIR to study its composition after the last cycle.

Results

Characterization of the sticky rice solution

The dried sticky rice paste was first examined by

SEM immediately after water evaporation (Fig. 1).

The dried gelatinized sticky rice presents an uneven

structure formed by a film with several granular

nodules tightly packed and clustered into compound

grains surrounded by some protein which may

Figure 1 SEM images of the

sticky rice. a 20009;

b 50009.

10222 J Mater Sci (2019) 54:10217–10234

influence the carbonation rate of lime putty, similar

as observed in the literature [4] (Fig. 1).

ATR-FTIR analysis of the dried sticky rice paste

showed absorbance bands that can be attributed to

rice starch (IRUG-ICB00021). The absorbance band at

847 and 761 cm-1 is attributed to the C-O group from

the glucose anhydride ring, and the absorption bands

at 3271 and 1654 cm-1 can be attributed to the

absorbance of the –OH groups in the starch [15, 48].

According to the latter reference this rice starch

spectrum can be attributed to amylopectin (supple-

mentary Information).

A red–brown color was very evident after the

addition of the iodine–KI reagent to the sticky rice

solution [38] which clearly indicates that the sticky

rice starch used was mainly composed of amy-

lopectin (supplementary information).

Rheological analysis shows that the sticky rice

suspension presents a constant complex viscosity

between 0.002 and 0.001 mPa s, similar to water

(supplementary information), and significantly lower

viscosity than that of other consolidation products

used in cultural heritage such as PEG (poly(ethylene

glycol)) [49].

The surface tension of sticky rice was 69.28

(± 0.81) mN/m and very similar to that of water

(69.99 (± 0.5) mN/m) (Fig. 2).

Consolidation effectiveness

The apparent and open porosity of treated and con-

trol samples was calculated after 24 h of total

immersion in water, and the actual pore size distri-

bution was measured by MIP (Table 1). It can be seen

that all treatments slightly reduced both apparent

and open porosity of the mortar, although the high

standard deviation, particularly for the LAQ samples,

makes this decrease not statistically significant. The

porosity reduction was confirmed by the MIP data;

the highest decrease in porosity corresponds to the

samples treated with sticky rice and nanolime, SR-

LAQ (* 22.9% decrease), followed by those treated

with nanolime only, LAQ, and sticky rice only, SR

(13.14% and 12.8%, respectively). This can be attrib-

uted to the higher amount of consolidant product

remaining in the samples (SR-LAQ) after solvent

evaporation, corresponding to 0.605 g (0.55 g of

sticky rice and 0.055 g of LAQ), while for SR about

0.55 g remained whereas only 0.055 g remained of

LAQ (both SR-LAQ and LAQ introduced the same

amount of nanolime).

The pore size distributions of treated and control

samples are shown in Fig. 3. The MIP curves clearly

show that the reduction in porosity produced by all

treatments can be attributed to a decrease in the

number of pores with diameters between 2 and

11 lm. The nanolime only (LAQ) and sticky rice–

nanolime (SR-LAQ) treatments also reduced the

population of pores with diameters between 0.01 and

0.3 lm, the latter treatment yielding a slightly more

marked decrease. LAQ treatment also slightly

reduced the population of pores with diameters

between 0.5 and 1 lm.

The water contact angle of treated samples was

carried out to determine if any hydrophobic proper-

ties were induced to the mortar by the sticky rice and

the sticky rice–nanolime treatments. The static con-

tact angle (h) on samples treated with LAQ is 0.

Figure 4 shows the static contact angle (h) on the

surfaces treated with SR and SR-LAQ. In both cases,

the application of sticky rice increases theFigure 2 Analysis of the pendant surface angle of: a water

(69.99 mN/M); b sticky rice solution (69.28 mN/m).

Table 1 Calculated Apparent

porosity (% g/g) and open

porosity (% cm3/cm3), and

porosity by Hg intrusion (%)

CO LAQ SR SR-LAQ

Apparent porosity % w/w 12.22 (± 0.84) 12.16 (± 1.09) 12.05 (± 0.28) 11.73 (± 0.19)

Open porosity % v/v 17.26 (± 0.56) 17.01 (± 1.71) 16.82 (± 0.63) 16.88 (± 0.14)

Porosity by Hg intrusion (%) 18.8 16.33 16.44 14.46

J Mater Sci (2019) 54:10217–10234 10223

hydrophobicity of the treated surface. The static

contact angle on SR samples is 106� (± 4.6), which

confirms the surface hydrophobicity; while the static

contact angle on SR-LAQ is 66� (± 5.41), suggesting

that the hydrophobicity of the treated surface

decreases when the application of sticky rice is fol-

lowed by that of nanolime. The different hydropho-

bicity of the treated surfaces is likely to affect the

contact angle between these and the mercury, which

in this study was taken to be 140�. This has to be

considered when interpreting the results of the MIP

measurements shown in Fig. 4 and a detailed study is

required in order to address this issue.

The capillary water absorption (WAC) of treated

samples is shown in Fig. 5 where it can be seen that

all treated samples show a reduced water absorption

(Table 2). The highest decrease was observed for

samples treated with sticky rice only (SR), followed

by those treated with sticky rice and nanolime (SR-

LAQ) and those treated with nanolime only (LAQ).

Table 2 also shows that all treated samples absorb

slightly less water when all capillaries are filled,

although the high standard deviation makes the

results not statistically significant.

Control and LAQ samples reached asymptotical

absorption values after 20 min of contact with water,

while samples treated with sticky rice only (SR) and

sticky rice and nanolime (SR-LAQ) needed approxi-

mately 5 h (Fig. 5), which clearly confirms that the

sticky rice reduces the speed of water absorption by

capillarity.

The drying curves are shown in Fig. 6. All treated

samples present slightly different initial drying rate

(from 0 to 25 h) and similar final drying kinetics

(from 35 to 50 h). LAQ presents slightly slower initial

drying rate followed by SR and SR-LAQ (Table 2).

All treated samples were completely dry after 50 h,

similarly to control samples. This confirms that the

application of sticky rice or nanolime does not

interfere significantly with the water evaporation of

the wet samples, which is desirable as this will

Figure 3 Differential volume of intruded mercury versus pore

diameter of samples treated with LAQ, SR and SR-LAQ.

Figure 4 Static contact angle

images after 1 s. a SR

(106� ± 4.6); b SR-LAQ

(66� ± 5.4).

Figure 5 WAC curves of control and treated samples.

10224 J Mater Sci (2019) 54:10217–10234

reduce deterioration mechanisms such as biodeteri-

oration or salt damage [50].

The results of the Scotch Tape Test (STT) are shown

in Table 3. All treatments yielded decreasing values

of removed material (21.9–5.77 mg/cm2). These

results confirm that all surfaces are more compact

after treatment (DW & 74.1–96%). The samples trea-

ted only with sticky rice and those with sticky rice

and nanolime showed a higher increase in surface

cohesion (DW & 96% and DW & 93%, respectively)

than those treated by nanolime only (DW & 74%).

Drilling resistance results for treated and untreated

samples are shown in Fig. 7. The force required to

drill a hole at constant rotation and lateral feed rate is

known to correlate with the compressive strength of

the material [46]. The samples treated with nanolime

only (LAQ) show a slightly increased drilling resis-

tance (F * 0.13 (± 0.19) N, DF * 62%) within about

6–8 mm from the surface, which is in line with the

results of previous research [24]. Drilling resistance of

the samples treated with sticky rice (SR) and with

sticky rice and nanolime (SR-LAQ) is higher than

those of the control and of that treated only with

nanolime, and they have a fairly constant value

throughout the drilling depth (Fig. 7). It is important

to note that during the application of the sticky rice,

in both SR and SR-LAQ samples, the solution reached

the opposite side of the cubic sample, indicating that

it is likely that the internal porosity of the sample was

Table 2 Water absorption and drying characteristics

Parameter CO LAQ SR SR-LAQ

W. absorption coefficient (10-3 g/cm2 s0.5) 19.1 (± 0.6) 15.1 (± 0.8) 7.0 (± 2.5) 10.1 (± 2.0)

W. absorbed at asymptotic value (g) 10.32 (± 0.70) 10.17 (± 0.92) 9.59 (± 0.80) 10.05 (± 0.10)

W. absorbed 24 h immersion (g) 10.32 (± 0.70) 10.17 (± 0.92) 9.59 (± 0.80) 10.05 (± 0.10)

Initial drying rate (10-3g/cm3 h) 9.7 (± 0.2) 9.5 (± 0.3) 7.7 (± 0.3) 7.5 (± 0.3)

Final drying rate (10-3g/cm3 h) 2.7 (± 0.3) 3.4 (± 0.2) 3.1 (± 0.4) 3.3 (± 0.4)

Time for total drying (h) ± 50 ± 50 ± 50 ± 50

Values in parentheses are standard deviation calculated from the three samples. W (water)

Figure 6 Drying curves of control and treated samples.

Table 3 Scotch Tape Test (STT): experimental results

Released material (mg/cm2) DW (%) SD

CO 84.59 – 12.01

LAQ 21.9 74.12 6.09

SR 3.04 96.41 3.5

SR-LAQ 5.77 93.18 5.6

Scotch area: 3 9 1.5 cm; SD (standard deviation of released

material)

Figure 7 DRMS of treated and untreated samples.

J Mater Sci (2019) 54:10217–10234 10225

covered by a sticky rice film. DRMS patterns indicate

that sticky rice provides a deeper consolidation of the

mortar independent of the use of nanolime in com-

bination with it. Since both LAQ and SR-LAQ sam-

ples received the same amount of nanolime particles,

DRMS patterns also suggest that the nanolime

introduced in the SR-LAQ samples could have been

distributed more homogeneously throughout the

pore structure rather than being accumulated in the

outer 6–8 mm. Further research is required to eluci-

date these hypotheses.

SEM images show morphological differences

between samples treated with sticky rice only and

those treated with sticky rice and nanolime (Fig. 8).

SR samples show that the sticky rice is homoge-

neously distributed as a film over the mortar grains

(Fig. 8a, b). In contrast, SR-LAQ samples show that

the calcite crystals of LAQ grow embedded in this

film as they crystallize upon carbonation (Fig. 9c, d),

as reported in the literature [4, 15]. However, the

morphology and size of those calcite crystals were

not reduced in size, as reported in studies of both

ancient and recent Chinese lime–mortars with sticky

rice [4, 9, 10, 17–19]. The calcite crystals were found to

be similar to the ones observed in preliminary studies

with the same nanolime [24].

Colorimetry results (Table 4) show that the treat-

ments involving nanolime (LAQ and SR-LAQ)

caused a whitening effect on the surface, with both

DE* and DL* values above 5, especially for samples

treated with only nanolime (LAQ). The increase in

whitening is attributed to the accumulation of nano-

lime particles on the surface, as described in previous

studies [24]. In contrast, SR samples do not show any

aesthetic alterations.

Durability of the consolidation treatments:comparison between AWT-1 (@50 �C),AWT-2 (@30aC) and wet–dry cycles

Scotch Tape Test

Results of the STT show that the surface cohesion

obtained during the consolidation treatment,

decreased significantly for SR and SR-LAQ samples

in the case of the 50 �C weathering and somewhat

Figure 8 SEM images of

a SR at 10,0009; b SR at

100,0009; c SR-LAQ at

2,0009; d SR-LAQ at

10,0009. Red arrow shows

location of the magnified

crystals in the sample.

10226 J Mater Sci (2019) 54:10217–10234

less for the 30 �C regime and wet–dry cycles

(Table 5). The decrease is more pronounced for

samples treated only with sticky rice. This can be

explained by the fact that the applied nanolime

treatment would protect somewhat the sticky rice

film as seen in SEM. Control samples confirm that

AWT cycles did not affect the surface cohesion of the

mortar. Wet–dry cycling reduced even less the sur-

face cohesion.

Water contact angle

The water contact angle measured on the samples

after AWT-1 shows that the hydrophobicity of the

samples treated with SR and SR-LAQ was signifi-

cantly reduced (Fig. 9). The contact angle is 0� on the

sample treated with SR-LAQ, and 20.6� (± 3.14) on

the sample treated with SR (Fig. 9), clearly suggesting

that the sticky rice, which was responsible for the

hydrophobic effect was lost during this aging pro-

cess. However, for the AWT-2 conditions, the water

contact angles of samples treated with SR and SR-

LAQ are 80.16� (± 3.18) and 69.26� (± 8.83), respec-

tively, indicating that the sticky rice is still present on

the sample surface. While the static contact angle

clearly decreased in SR samples from approximately

106� to about 80� (before and after AWT-2) there was

a minimum change for the SR-LAQ samples, from

approximately 66� to about 69� (before and after

AWT 2), suggesting that the nanolime treatment

protected the sticky rice. After wet–dry cycling, the

sample treated only with sticky rice retained an angle

of about 71�, while that treated also with nanolime

decreased to about a 43� angle.

Drilling resistance

For both samples, SR and SR-LAQ, drilling resistance

significantly decreased after AWT-1 (50 �C), which is

in line with the STT and the contact angle results

described above. The drilling resistance of SR sam-

ples was reduced to values close to those of the

Table 4 Chromatic alterations

for treated samples DL* Da* Db* DE*

LAQ 11.94 (± 0.97) - 1.25 (± 0.26) - 6.96 (± 1.04) 13.87

SR 2.19 (± 2.35) - 0.56 (± 0.24) - 1.20 (± 0.74) 2.55

SR-LAQ 7.99 (± 1.72) - 0.59 (± 0.48) - 1.91 (± 0.83) 8.23

Mean values determined 30 measurements

Figure 9 Static contact angle of treated samples after: Top line:

Left sample treated with SR (20.6� ± 3.14) after AWT-1

(T = 50–60 �C); center sample treated with SR (80.2� ± 8.83)

after AWT-2 (T = 40–30 �C); Right sample treated with SR-LAQ

(69.26� ± 8.83) after AWT-2 (T = 40–30 �C); Bottom line:

Samples after wet–dry cycling, Left, sample treated with SR

(71.1� ± 6.12); Right, sample treated with SR-LAQ

(42.8� ± 5.15).

J Mater Sci (2019) 54:10217–10234 10227

control sample (Fig. 10). In contrast, the drilling

resistance of SR-LAQ remained similar to that

recorded before the AWT-1 within about 2 mm from

the surface and decreased to values slightly above

those of the control throughout the remaining drilling

depth. The more durable consolidation effect on the

SR-LAQ samples compared to SR samples could be

attributed to the effect of nanolime on this sample. In

the case of the AWT-2 (30 �C), both samples also

show a decrease in resistance, following the same

pattern as for STT tests, suggesting that the starch

also partially degrades under these conditions. In the

case of wet–dry cycles, drilling profiles are similar to

those observed for samples exposed to AWT-2 (at

T = 30–40 �C) confirming that the degradation of

starch can also occur at room conditions if sufficient

water is available. In all cases, the decrease in the

drilling resistance is significantly more pronounced

in SR samples than in SR-LAQ samples, confirming

the results obtained for the contact angle and STT.

Wet–dry cycling showed a similar decrease in the

two samples.

Results of STT, contact angle and DRMS clearly

show that the consolidating effect of the sticky rice is

lost during the AWT-1 cycling and can be attributed

to the higher temperatures (T = 60–50 �C), which

may have degraded the sticky rice starch due to the

presence of water and heat, especially at tempera-

tures above 55 �C where starch undergoes an irre-

versible gelatinization process [51], where the water

dissolves starch granules by breaking down the

intermolecular bonds of starch molecules [52]. This

process turned the sample a yellowish color (DE *3). Therefore, for both samples (SR and SR-LAQ), the

sticky rice is partially solubilized and washed out of

the sample during the AWT-1 aging.

These results suggest that the degradation of sticky

rice is higher when it is applied on its own rather

than when applied in combination with nanolime

treatment, where the calcite crystals grow up

embedded in the sticky rice film (as shown in

Fig. 8d). This might increase the adhesion to the

mortar as both calcite and starch film may bond to

the grains and matrix physically and chemically thus

making the sticky rice slightly more resistant to sol-

ubilization. In contrast, when the sticky rice solution

is applied without nanolime, it forms a film over the

substrate grains (Fig. 8b), which may suggest that

this film only adheres to the mortar grains physically,

thus, more likely to solubilize as it does not bond

chemically to the mortar.

Fourier-transform infrared spectroscopy (FTIR)

To confirm the solubilization of the sticky rice, ATR-

FTIR analysis was carried out on the water used for

the wet–dry cycles. This analysis clearly shows that

the water contains some small traces of sticky rice

confirming the partial dissolution of some starch in

the water. The absorbance band between 1000 and

1500 cm-1 is attributed to sticky rice (supplementary

information).

Discussion

The present study aimed to address the effect that

sticky rice (SR) had on mortar samples when applied

as both a consolidant and pre-treatment before

Table 5 Results of the Scotch Tape Test (STT) after the weathering processes

Released material

(mg/cm2) before

weathering

DW (%)

before

weathering

Released

material (mg/

cm2) after AWT1

DW (%)

after

AWT1

Released

material (mg/

cm2) afterAWT2

DW (%)

after

AWT2

Releasedmaterial

(mg/cm2) after

wet–dry

DW (%)

after

wet–dry

CO 84.59 (± 12.01) – 83.12 (± 15.7) – 83.32 (± 13.1) – 82.98 (± 12.2) –

SR 3.04 (± 3.5) 96.41 18.64 (± 3.21) 77.6 11.1 (± 1.4) 86.65 9.92 (± 0.98) 88.07

SR-LAQ 5.77 (± 5.6) 93.18 12.88 (± 1.08) 84.51 8.74 (± 0.88) 89.49 10.02 (± 1.45) 87.73

Scotch Tape Test area: 3 9 1.5 cm; in parenthesis are the standard deviations

cFigure 10 DRMS of samples before and after weathering cycle:

Top line: Left: SR after AWT-1 (T = 50–60 �C); Right: SR-LAQafter AWT-1 (T = 50–60 �C); Center line: Left: SR after AWT-2

(T = 30–40 �C); Right: SR-LAQ after AWT-2 (T = 30–40 �C);Bottom line: Left, SR after wet–dry cycles); Right: SR-LAQ after

wet–dry cycles.

10228 J Mater Sci (2019) 54:10217–10234

J Mater Sci (2019) 54:10217–10234 10229

applying the nanolime. It was shown that porosity

changed with a significant decrease in larger pores

(between 2 and 11 lm) and a minor decrease in two

ranges of smaller pores (0.01–0.3 lm and 0.5–1 lm).

Water absorption results reflected the hydrophobic

effect of the samples treated, or pre-treated with SR,

while the initial drying rate for all treated specimens

was slower than that of the control, no significant

difference was observed between the LAQ samples

and those treated with SR.

The Scotch Tape Test and the Drilling Resistance

measurements showed that pre-treatment with SR

resulted in both an increased surface cohesion and

penetration depth. Since the amount of nanolime

introduced was the same for both LAQ by itself or

when applied over the SR, the DRMS patterns sug-

gest that nanolime could be distributed more homo-

geneously throughout the pores in the sample when

applied after a pre-treatment with SR, as in this

sample the nanoparticles did not accumulate in the

outer 6–8 mm. SEM images show that nanolime

particles are completely embedded in the sticky rice

film in the pores, where they crystalized. Both DRMS

and SEM results support the hypothesis that the

sticky rice film could attach nanolime particles to the

film during the application of nanolime, reducing the

back-migration of the nanoparticles thus contributing

to their in-depth deposition and providing a

homogenous distribution throughout the sample.

This topic needs further study to determine the

influence of sticky rice on the penetration of nano-

lime. Finally, the colorimetric results show that while

consolidation with LAQ resulted in a significant

whitening effect (DE* 13.87), when pre-treated with

sticky rice it was significantly reduced (DE* 8.23),

while application of sticky rice by itself was below

visual detection level.

Previous studies where the sticky rice was added

to lime for mortars reported that between 3 and 5%

w/w of SR accelerated the setting and hardening (i.e.,

increased compressive strength) and bonding capa-

bility, although unit weight and water resistance

were slightly reduced [16]. Another study [3] showed

that consolidated samples could resist a 2 months

water immersion without falling apart as the control

samples did. This study focused on the effect that

sticky rice would have as a consolidant, either by

itself or followed by a nanolime consolidation treat-

ment applied to low resistance mortars. This is a

significant difference in approach than the traditional

Chinese application and therefore previous studies

cannot be compared directly. The weathering cycles

used in this study to test the resistance of the treated

samples showed that they are susceptible to wet–dry

cycling damage, particularly if subjected to higher

temperature changes since the gelatinization process

of the amylopectin occurs at temperatures above

55 �C [50]. During this degradation process, the

sticky rice samples acquired a yellowish tinge, which

was measured by colorimetric analysis (DE * 3).

Future studies should also consider the reapplication

of sticky rice solution after the nanolime

consolidation.

SEM images of the treated samples show mor-

phological differences between the two treatments

involving sticky rice (SR and SR-LAQ). SR samples

show that sticky rice forms a film over the substrate

grains, while the SR-LAQ samples show calcite

crystals growing up embedded in the sticky rice film.

However, the growth of the calcite crystals in the

presence of sticky rice was similar to the calcite

crystal size reported in previous studies [24] con-

firming that they did not reduce in size as happens

when sticky rice and lime are mixed together during

the manufacture of the mortar [4, 15]. In this case, the

amylopectin was found to play a crucial role in the

carbonation process of Ca(OH)2 inhibiting the growth

of the CaCO3 crystals so that only smaller calcite

crystals developed forming a dense and compact

microstructure that reduces the penetration of water

and increases their durability [15, 18]. This is a sig-

nificant difference that reflects the disparities in

approach, the Chinese studies focused on mortar

preparation, while this one concentrates on their

consolidation.

The presence of the sticky rice may increase bio-

colonization of the treated area, since it is close to the

surface, and needs to be considered. In the traditional

method where the sticky rice was added during the

lime quenching, bacteria were eliminated by the

released heat [7, 10]. In the present situation, where

the SR is applied directly to the object biocolonization

may develop faster, especially in areas where mois-

ture prevails. However, since wet–dry cycling of the

treated object will first eliminate the SR from the

surface, the biocolonization may be retarded. This is a

point that needs to be taken into account in future

research.

Further studies with sticky rice are needed to

understand the deterioration process of the sticky rice

10230 J Mater Sci (2019) 54:10217–10234

and if damage could be caused by the swelling that

occurs during starch hydrolysis. Additionally, the

long-term biodegradation effect on this polysaccha-

ride needs to be evaluated since it can be a source of

nutrients for microorganisms. Although more

research is required to evaluate the promising con-

solidation results obtained, such as actual depth of

penetration and repeating the application of sticky

rice after the nanolime, it could be preliminary con-

cluded that sticky rice nanolime consolidation could

be used to repair weathered materials in very dry

environments (e.g., Mediterranean Eastern countries)

or in interior spaces where no liquid water is to be

found.

Conclusions

The promising results from this innovative research

showed that the use of sticky rice alone or in com-

bination with LAQ nanolime yields a higher degree

of consolidation than that attained by a simple

nanolime consolidation. However, the consolidation

effectiveness of sticky rice decreases when the treated

samples are exposed to weathering processes. More

research needs to be carried out to fully understand

the sticky rice weathering process to optimize its

consolidation mechanism.

This study has shown that:

• The sticky rice solution has low viscosity which

favors good penetration into the porous substrate.

FTIR and the iodine test confirm that the sticky

rice starch is composed mainly of amylopectin.

The morphological properties of the dried gela-

tinized sticky rice show that it presents an uneven

structure formed by a film with several granular

nodules tightly packed and clustered into com-

pound grains.

• All treated samples, either with nanolime (LAQ),

sticky rice (SR) or their combination (SR-LAQ),

had a lower porosity. The highest porosity

decrease was observed for samples treated with

SR-LAQ (* 22.9% decrease), followed by those

treated with SR (12.8%). MIP results clearly show

that the reduction in porosity corresponds to the

filling of pores with diameters between 2 and

11 lm in all treatments. Both treated samples

involving nanolime, i.e., LAQ and SR-LAQ, also

showed a reduction in the smaller pore range with

diameter between 0.01 and 0.3 lm and confirming

that LAQ can partially fill these finer pores.

• A significant increase of surface cohesion. The

samples treated with only sticky rice showed the

highest increase in the superficial cohesion fol-

lowed by samples treated with sticky rice and

nanolime.

• A certain degree of hydrophobicity was imparted

to the sample surface by the application of the SR,

but it decreases when nanolime is applied subse-

quently. Further studies are required to elucidate

this behavior.

• The drilling resistance increases for all treated

samples, especially those pre-treated with sticky

rice, where the treatment certainly reached 2 cm

(half the length of the sample reached by the

drilling) while those treated only with LAQ

merely reach 6 mm. The increase in the drilling

resistance is fairly constant throughout the dril-

ling depth, confirming that sticky rice starch

increases the mechanical resistance of samples

providing a deeper consolidation of the mortar

independent of the use of nanolime in combina-

tion with it. The increased durability of the SR-

LAQ treatment could be attributed to the sticky

rice which apparently improved nanolime distri-

bution throughout the sample. Further research is

required to elucidate this hypothesis and to study

the total penetration depth of the sticky rice

independent of the use of nanolime in combina-

tion with it.

• Colorimetric results show that both samples

involving nanolime (LAQ and SR-LAQ) caused a

slight whitening of the surface with both DE* andDL* values above 5.

• The surface cohesion, hydrophobicity and drilling

resistance of sticky rice samples significantly

decreased after exposure of the samples to the

AWT-1 (T = 40–60 �C) weathering which may be

attributed to the irreversible starch gelatinization

process.

• A lower decrease in the consolidation was notice-

able after exposure of the samples to the AWT-2

(T = 40–30 �C) and similar results were obtained

after wet–dry cycles. These results confirm that

starch still degrades at medium or room temper-

atures after several moisture cycles.

• FTIR analysis of the water from wet–dry cycles

shows that small amounts of sticky rice starch was

dissolved in the water.

J Mater Sci (2019) 54:10217–10234 10231

• The decrease in surface cohesion, hydrophobicity

and drilling resistance after any of the weathering

processes tested is more pronounced in SR sam-

ples than in SR-LAQ. This could be attributed to

the presence of the calcite crystals growing on the

sticky rice film which might increase the adhesion

to the matrix of both calcite and starch film, as

both may bond to the matrix physically and

chemically due to the calcite–calcite bonding,

reducing the solubilization of the sticky rice film.

Acknowledgements

This research has been funded by the Vice Chancel-

lor’s Scholarship within the Doctorate Program by

Sheffield Hallam University (UK) and through the

Cantor Mobility Scholarship Scheme for my research

stay at the Smithsonian Institution (USA).

Electronic supplementary material: The online

version of this article (https://doi.org/10.1007/s108

53-019-03618-1) contains supplementary material,

which is available to authorized users.

Open Access This article is distributed under the

terms of the Creative Commons Attribution 4.0

International License (http://creativecommons.org/

licenses/by/4.0/), which permits unrestricted use,

distribution, and reproduction in any medium, pro-

vided you give appropriate credit to the original

author(s) and the source, provide a link to the Crea-

tive Commons license, and indicate if changes were

made.

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